People in the Romanian capital demonstrate against the ordinance.
Photograph: Daniel Mihailescu/AFP/Getty Images

Tens of thousands of people have taken to the streets in Romania to protest against an emergency ordinance passed on Tuesday night that decriminalises some offences, including official misconduct in cases where the financial damage is less than 200,000 lei (£38,000).

Critics of the ordinance, introduced by a government that won parliamentary elections in December, say it could reverse an anti-corruption fight in Romania that has drawn widespread praise internationally. Additional laws could be issued in the coming days, further eroding ministers’ accountability, according to the government’s opponents.

In a post on Facebook, the Romanian president, Klaus Iohannis, who has opposed the emergency ordinance and who last month joined protesters on the streets, wrote: “Today is a day of mourning for the rule of law.”

News of the ordinance passing broke at about 10pm local time, and by midnight local media estimated that 12,000 people were demonstrating in Bucharest and 10,000 in other cities across the country, despite freezing temperatures. There were shouts of “Thieves” and “You won’t get away with it.”

“I don’t normally protest but I just felt such a sense of rage,” said Alexandra Boeriu, a 35-year-old NGO worker who protested outside the main government building in Bucharest. “I was young, but I did live through communism and I know what this is. I don’t want this for my kids. There are a lot of people protesting who want to have a future in this country. It feels like someone has died.”

Klaus Iohannis: ‘Today is a day of mourning for the rule of law.’ Photograph: Patrick Seeger/EPA

“You can’t just publish laws in the night and say shut up, we won the election, you have no rights,” said Dragos Stanca, 42, a digital media entrepreneur who was also demonstrating. “It’s a very dangerous precedent. If this is the kind of procedure they do, we wonder what’s next. It looks like anything is possible.”

Protests have been held for the past two Sundays over the government’s plans, which would also grant amnesty to those sentenced to jail terms shorter than five years in relation to certain crimes and halve prison terms for those with young children, pregnant women, or those over 60.

The EU has weighed in on the controversy, warning the government not to backtrack on anti-graft measures. “The fight against corruption needs to be advanced, not undone,” European commission head Jean-Claude Juncker and his deputy, Frans Timmermans, said in a joint statement. “We are following the latest developments in Romania with great concern.”

Government officials have suggested the laws were aimed at easing overcrowded conditions in prisons, but few observers are convinced, and many believe the real purpose is to help convicted politicians, or those who are now under investigation, escape justice.

Romania, an enthusiastic EU member for a decade and a major recipient of EU funds, remains one of Europe’s most corrupt countries: a report by the IPP thinktank released late last year found that of the 588 MPs elected in previous elections in 2012, 89 – or 15% – were either under investigation for graft, had already been convicted, or chose to step down for other positions.

Hundreds of officials have been imprisoned in recent years, and more than 2,000 files are under investigation in relation to abuse of office, according to prosecutors.

“With this government I feel insecure and the new law which favours corruption is a huge offence and an attack on us, the regular people,” said Alberto Grosescu, a 29-year-old photographer who took to the streets on Tuesday night immediately after the ordinance was announced.

Grosescu predicts that many more people will come out to protest. “I’m pretty sure that tonight there is going to be over 100,000 people in Bucharest, maybe over 200,000 across the whole country,” he said.

Those protesting hope that their actions could still lead to change.

“They have to, I don’t think politicians can ignore so many people, but we have to stand up and continue to stand up,” said Boeriu.

Thousands of people gathered outside the main government building in Bucharest on Wednesday morning. It is thought that protests later in the day could be the biggest since the fall of communism in 1989.